Electrically insulating material, method for the preparation thereof, and insulated objects comprising said material

ABSTRACT

An electrically insulating material including a continuous phase of a thermoplastic polymer and an additional phase incorporated therein of a liquid or easily meltable dielectric in the form of a wholly or partly interpenetrating network, and where the weight ratio of the polymer to dielectric is between 95:5 and 25:75.

This application is a 371 of PCT/DK98/00382, filed Sep. 9, 1998.

An electrically insulating material, method for the preparation thereof,and insulated objects comprising said material.

The present invention relates to an electrically insulating materialcomprising a thermoplastic polymer and a dielectric.

It is known to insulate high-voltage DC cables with paper impregnatedwith dielectric oil. The preparation of such insulated cables iscumbersome and time-consuming, as it comprises a number of steps, suchas wrapping the paper round the electrical conductor, drying,impregnating the paper under heating, and cooling the insulation toambient temperature. Such cables can also be used for alternatingcurrent.

By using the known insulations, local charge effects which may causebreakdown can be avoided, but the resulting cables are sensitive toquenching, and the operating temperatures should not exceed about 80° C.

GB patent No. 1.371.991 discloses an insulation material, which isprepared by impregnating a porous, electrically insulating polymericfilm with a dielectric fluid, followed by a heat-shrinkage of thepolymeric film in view of encapsulating the dielectric fluid. The use ofthe known insulation material for insulating e.g. high-voltage cables isslow as, like the first-mentioned insulating method, it presupposes thatthe impregnated polymeric film is wound round the electrical conductor.

It is also known to insulate high-voltage AC cables with an insulatinginsulation layer prepared by extrusion of a polymer, such aspolyethylene or cross-linked polyethylene.

It has not been possible to use such insulation layers of a polymer forinsulating high-voltage DC cables, i.a. because during cooling toambient temperature at operating conditions, local charge effects areaccumulated, thus entailing risk of breakdown.

Also, it is known to use various types of gels for insulation and/orfield equalization.

U.S. Pat. No. 4,943,685 discloses the use of a gel formed from a lightlycross-linked polymer and insulating fluid, such as a mineral oil, forinjecting into e.g. cable splices or cable shoes, so that the gel fillsout the void around the conductor and acts as insulation.

U.S. Pat. No. 5,218,011 discloses the use of a gel compositioncomprising a fluid, a thickener, and a water absorbent polymer forincorporation as filler in cavities and in electrical cables. The mainpurpose of the presence of such a gel is to prevent entry of water,which i.a. is achieved in that the gel itself forms a barrier. If waterdoes enter past this barrier, the water absorbent polymer is activated,and the water is absorbed. This type of gel is mainly used in connectionwith low direct voltages.

WO 86/01634 discloses use of a gelloid composition comprising a polymer,in which a fluid is dispersed, and and optionally a filler, for fieldequalization in connection with electrical devices. The composition isespecially well-suited for use at high voltages.

It is a common feature of these types of gels that they have nomechanical strength, for which reason they are most unfit for formationof a dimensionally stable insulation layer. Typically, it is the purposeof the gel to act as a mass which displaces air, as air is poorlyinsulating. As regards the gels mentioned in U.S. Pat. No. 5,218,011 andWO 86/01634 there is the additional disadvantage that the particular gelin itself has no particular insulating effect, for which reason anadditional insulation layer must typically be used.

JP 8302113A discloses the use of an ethylene-propylene rubber compoundedwith at least one compound selected from polybutene, polybutadiene,polysioprene, and butyl rubber, inorganic fillers, and organic peroxidesfor the preparation of an insulation material without use of added oil.

Finally, WO 96/27885 discloses use of a composition comprising apolypropylene polymer or copolymer, polyethylene wax, and coatedmagnesium hydroxide as insulation or outer sheath for wires and cables.Such a composition is easily extrudable, and the wax content ensures asmooth and wear-resistant surface.

Use of the above composition for high-voltage is, however, inexpedientbecause of the high content of magnesium hydroxide added in view of thefire retarding effect of the substance.

It is the object of the present invention to provide a material whichpossesses sufficient insulating capacity for it to be used for both DCand AC insulation in connection with high-voltage, and which is easilyconverted so as to form a desired insulation layer.

This object and other objects, which will be described in the following,are obtained with the insulation material according to the invention,which is characterized in that the thermoplastic polymer forms acontinuous phase incorporating an additional phase of a liquid or easilymeltable dielectric in the form of a wholly or partly interpenetratingnetwork, and that the weight ratio of polymer to dielectric is between95:5 and 25:75.

When using an electrically insulating material for e.g. high-voltageinsulation, a temperature increase normally occurs, whereby thedielectric, if not already liquid, melts. Hereby a structure emergescomprising a solid network of polymer filled with liquid dielectric,which thereby gets to act as a mobile phase in the solid polymernetwork.

The presence of this mobile phase seems to prevent local charge effects,which in the known materials may cause breakdown, from arising, andwithout this phase inexpediently influencing the main structure andconsequently the strength of the insulation material.

Examples of useful thermoplastic polymers include polyolefines, acetatepolymers, cellulose polymers, polyesters, polyketones, polyacrylates,polyamides, and polyamines. The polymers may be homo-, co- orter-polymers. As co-monomers use can be made of various compounds withfunctional groups, such as epoxides, vinyls, amines, anhydrides,isocyanates, and nitriles. Mixtures of two or more polymers can also beused.

To avoid exudation of dielectric after the preparation of the insulationmaterial, it is preferred to use low-crystalline polymers.

The liquid dielectric is preferably a mineral or synthetic oil, or amixture of both. Low-viscosity as well as high-viscosity oils may beused.

Examples of use as dielectric oils include polyisobutylene, naphthenic,polyaromatic, and alpha-olefine containing oils, as well as siliconeoils.

Examples of easily meltable dielectrics are wax and low molecularpolymers.

In this context, the expression “easily meltable” should be taken tomean that the dielectric melts/softens at a lower temperature than themelting/softening temperature for the thermoplastic polymer.

The invention also relates to a method for the preparation of theelectrically insulating material described above. This method ischaracterized in that the thermoplastic polymer and a liquid or easilymeltable dielectric in a weight ratio from 95:5 to 25:75 of polymer todielectric are mixed under heating to a sufficiently high temperaturefor melting both polymer and dielectric, that the mixture is optionallyformed to a shape, and that it is cooled to ambient temperature. Herebyan insulation material is obtained which is dimensionally stable attemperatures of use, and consequently can be used without cross-linkingas insulation material on e.g. high-voltage cables.

During the mixing and the heating of the thermoplastic polymer and theliquid or meltable dielectric, a liquid-in-liquid suspension isobtained, where the polymer as a result of its comparatively highviscosity predominantly forms a continuous phase, in which the liquiddielectric forms a similarly continuous, interpenetrating phase. It ispresumed that a corresponding backbone structure is obtained aftercooling the mixture to ambient temperature, however, with the differencethat the polymer after having again assumed solid state forms a networkcontaining a wholly or partly interpenetrating network of liquid ofsolidified dielectric.

It is understood that the said interpenetrating network is formed atmicroscopic level, and, as it is, is not comparable with network atmolecular level provided e.g. by cross-linking of polymer chains and/orformation of a gel structure.

The weight ratio of polymer to dielectric is, as mentioned, from 95:5 to25:75. Particularly preferred ratios are from 90:10 to 50:50, and inparticular from 90:10 to 75:25.

It may be advantageous to reinforce the polymer network in theinsulating material according to the invention by evoking in the saidmixture a cross-linking in the polymer. Such cross-linking can e.g. beobtained by radiation treatment or by admixing a cross-linking agent,e.g. in the form of a triallyl cyanurate, silanes or peroxides.

The mixture of polymer and dielectric can be added with one or moreadditives and/or fillers. For example, carbon black, titanium dioxide,wood powder or cellulose derivatives can be used for equalizingelectrical fields.

The temperature to which the mixture is heated depends on themelting/softening point of the thermoplastic polymer, and shouldpreferably lie more than 10° C. over this temperature. For α-olefines atemperature of up to 160° C. is typically used, and for e.g. polyamides,cellulose polymers, and polyketones a temperature up to 230° C.

The thermoplastic polymer and the dielectric can be mixed and heatedbatch-wise or continuously, e.g. using an extruder. The mixed mass canbe granulated and used as starting material for formation of desiredinsulation layers. For example, it can be extruded directly onto anelectrical conductor so as to form an insulation layer thereon, or by amulti-step extrusion of the electrically insulating material optionallyadded with carbon black or another additive. The additive can also beadded to the polymer prior to the mixing thereof with the dielectric.

Injection moulding, thermoforming or the like may also be used for theshaping.

The mixing and the heating as well as the extrusion onto a conductor mayalso take place in one step.

The invention further relates to objects, such as cables insulated withthe electrically insulating material described above. Such insulatedcables can be used for both direct current and alternating current,preferably for direct current, and at voltages from 220 V to 10 MV.Preferred uses are for voltages greater than 5 kV, as the material athigh field strengths is capable of maintaining its good electricalproperties.

The insulation material described can also be used for other insulatingpurposes, e.g. for insulating terminations, cable splices, cableterminals, transformer insulation, for the preparation of dielectriccomponents, for use in X-ray generators, and for other high-voltagepurposes.

In the following the invention is described in more detail withreference to the examples below.

EXAMPLE 1

40 parts by volume of naphthenic oil with a viscosity at 25° C. of 12 cpwere heated to 150° C. under stirring with a stirrer having a rotationalspeed of 30 rpm/min. Then 60 parts by volume of alpha-olefine containingpolymer with an MFI of 0.6 g/10 min and a melting temperature of 142° C.were added. Mixing was for 4 min at 150° C. The mixture thus obtainedwas cooled and granulated at ambient temperature. The granulate wasintroduced into an extruder and extruded in the form of a coating ontoan electrical conductor at a temperature of 140-160° C.

The insulating coating thus prepared was thermally stable andmechanically stable at temperatures up to about 80° C. The coatingconsisting of two interpenetrating networks did not exudate oil at atemperature of 80° C. and a superpressure of 1 bar.

By examining the breakdown strength of the insulating coating it wasestablished that this strength was at least as high as for an insulatingcoating consisting of oil impregnated paper.

EXAMPLE 2

An insulating coating was prepared on an electrical conductor by amethod corresponding to that described in example 1, but usingpolyisobutylene oil instead of a naphthenic oil.

The coating obtained had essentially the same properties as the coatingaccording to example 1.

EXAMPLE 3

An insulating coating was prepared on an electrical conductor by amethod corresponding to that described in example 1, with the exceptionthat 70 parts by volume of polymer and 30 parts by volume of oil wereused.

EXAMPLE 3a

An insulating coating was prepared on an electrical conductor by amethod corresponding to that described in example 1, however using 80parts by volume of polymer and 20 parts by volume of oil.

The coatings obtained had essentially the same properties as the coatingaccording to example 1.

Measurement of rate of local charging and decharging for the insulationmaterials prepared in examples 1, 3 and 3a was made by means of PulsedElectro Accoustic Method (PEA). Test specimens were prepared fromsemi-conductor and have a thickness of 2 mm. Charging is effected with20 kV DC voltage, and charging and decharging are for 24 hours.Measurements are made without impressed voltage on the test specimen.

Standard decharging rates for the materials from examples 1, 3 and 3aare stated in table 1 and compared with conventional AC PEX insulationand oil impregnated paper insulation.

PEX Oil/paper Polymer* Ex. 1 Ex. 3 Ex. 3a Unit Min. Min. Min. Min. Min.Min. Decharg- >500 200 >500 30 50 50 ing time *alpha-olefinic polymerused in examples 1-3 and 3a Table 1. Decharging rates for differentdielectrics.

EXAMPLE 4

An insulating coating was prepared on an electrical conductor by amethod corresponding to that described in example 1, but using 10 partsby volume of paraffinic wax with melting interval of 57-60° C. fromMerck, 80 parts of extrudable LDPE (from Dow), and 10 parts of powderedadditive consisting of wood with a maximum diameter of 65 μm.

The insulation material obtained has essentially the same properties asthe insulation in example 1.

EXAMPLE 5

An insulating coating was prepared on an electrical conductor by amethod corresponding to that described in example 1, but using 10 partsby volume of polycyclic oil with a density of 1.04/cm³, 89 parts ofethylene vinyl acetate (24% vinyl acetate) with an MFI of 3 g/10 min(2.16 kg/190° C., ASTM D1238), and 1 part of powdered additiveconsisting of alumina trihydrate (Apyral 40 from Nabaltec) with a grainsize diameter or about 1.5 μm.

The insulation material obtained has essentially the same properties asthe insulation in example 1.

EXAMPLE 6

An insulating coating was prepared on an electrical conductor by amethod corresponding to that described in example 1, but using 5 partsby volume of chemically pure oleic acid, 94.8 parts of ethylene acrylatewith 2% maleic anhydride (Lotader 2100 from Elf Atochem), and 0.2 partsof powdered additive consisting of chemically pure titanium dioxide.

The insulating material obtained has essentially the same properties asthe insulation in example 1.

EXAMPLE 7

15 parts by volume of epoxidized soybean oil were mixed with 85 parts ofLDPE cable insulation polyethylene, into which 1.5% of dicumyl peroxidehad been premixed. The mixing took place as described in example 1,however, mixing was at 135° C. The insulation material thus prepared wascross-linked by heating to 180° C. under pressure (10 bar).

The coating material obtained has essentially the same properties as theinsulation in example 1.

What is claimed is:
 1. An electrically insulating material comprising athermoplastic polymer and a dielectric having a weight ratio of polymerto dielectric between 95:5 and 25:75, wherein the thermoplastic polymerforms a network of solid polymer filled with the dielectric, thedielectric being a liquid or a solid that melts or softens at a lowertemperature than the thermoplastic polymer such that the dielectric actsas a mobile phase in the solid polymer network, said insulating materialmaintaining its dielectric properties when subjected to voltages greaterthan 5 kV.
 2. An electrically insulating material according to claim 1,wherein the thermoplastic polymer forms a continuous phase incorporatingthe dielectric in the form of a wholly or partly interpenetratingnetwork.
 3. An electrically insulating material according to claim 1,wherein the thermoplastic polymer is selected from the group consistingof polyolefins, acetate polymers, cellulose polymers, polyesters,polyketones, polyacrylates, polyamides, polyamines, and epoxides, or amixture of two or more of the group.
 4. An electrically insulatingmaterial according to claim 1, wherein the thermoplastic polymer islow-crystalline.
 5. An electrically insulating material according toclaim 1, wherein the dielectric is a liquid selected from the groupconsisting of mineral oil and synthetic oil.
 6. An electricallyinsulating material according to claim 5, wherein the dielectric is anoil selected from the group consisting of polyisobutylene oils,naphthalenic oils, alpha-olefinic oils, and silicone oils.
 7. Anelectrically insulating material according to claim 1, wherein thedielectric is a wax.
 8. An electrically insulating material according toclaim 1, wherein the weight ratio of polymer to dielectric is between90:10 and 50:50.
 9. An electrically insulating material according toclaim 8, wherein the weight ratio of polymer to dielectric is between90:10 and 75:25.
 10. An electrically insulating material according toclaim 1, wherein the material is thermally and mechanically stable at atemperature of 80° C.
 11. A method for the preparation of theelectrically insulating material of claim 1 comprising mixing thethermoplastic polymer and the dielectric at a weight ratio between 95:5and 25:75 of polymer to dielectric under heating to a sufficiently hightemperature for melting the thermoplastic polymer, optionally shapingthe mixture and cooling the mixture to ambient temperature.
 12. A methodaccording to claim 11, wherein the thermoplastic polymer is cross-linkedduring the mixing under heating.
 13. A method according to claim 12,comprising introducing a cross-linking agent into the mixture ofthermoplastic polymer and dielectric.
 14. A method according to claim11, comprising adding an additive to the mixture of thermoplasticpolymer and dielectric.
 15. A method according to claim 14, wherein theadditive is selected from the group consisting of carbon black, titaniumdioxide, aluminum hydroxide, cellulose derivatives and wood powder. 16.A method according to claim 11, wherein the thermoplastic polymer ismixed with an additive or a filler prior to being mixed with thedielectric.
 17. A method according to claim 11, wherein the mixture isshaped by extrusion.
 18. A method according to claim 17, wherein themixture is extruded as an insulation layer onto an electrical conductor.19. An electrically insulated object comprising an electrical conductorsurrounded by the electrically insulation material according to claim 1.20. An electrically insulated object according to claim 19 wherein theelectrical conductor is for voltages greater than 36 kV.
 21. Anelectrically insulated object according to claim 19, wherein theelectrical conductor is for voltages greater than 150 kV.
 22. Anelectrically insulated object according to claim 19, wherein theelectrical conductor is for voltages greater than 400 kV.
 23. A methodfor conducting electricity comprising: a) providing the electricallyinsulated object of claim 19; and b) passing a direct current throughthe object at a voltage greater than 5 kV.
 24. A method comprisinginsulating a cable with the electrically insulated material of claim 1.25. A method comprising insulating a cable assembly with theelectrically insulated material of claim
 1. 26. A method comprisingpreparing a dielectric component with the electrically insulatedmaterial of claim 1 and incorporating said component into a piece ofhigh-voltage equipment.
 27. An electrically insulating materialcomprising a thermoplastic polymer and a dielectric having a weightratio of polymer to dielectric between 95:5 and 25:75, wherein thethermoplastic polymer forms a network of solid polymer filled with thedielectric, said dielectric comprising a liquid phase or a solid phasethat melts or softens at a lower temperature than the thermoplasticpolymer and is able to act as a mobile phase in the solid polymernetwork.